Precipitating cold clouds are still a challenge not only for many car drivers but also for atmospheric modelers and observationalists. One reason is the enormous variability of hydrometeors ranging from supercooled liquid droplets to single ice crystals, snow aggregates, and rimed particles.

Active and passive remote sensors operating in the microwave are important tools to obtain information about these optically thick winter clouds. However, a fundamental requirement to interpret their measurements is a profound understanding of the hydrometeors’ radiative properties like characteristic absorption or scattering behavior.

In this talk I will show two examples, how ground-based multi-frequency microwave sensors can help to close existing knowledge gaps: In the first part, a simple methodology will be presented that allows to constrain the absorption properties of supercooled liquid water using long-term ground-based microwave radiometer data. These information help to improve existing liquid water absorption models which are essential for the development of liquid water retrievals.

In the second part I will show recent discoveries in triple-frequency cloud radar observations of snowfall. By combining snow scattering simulations with collocated triple-frequency radar observations and in-situ data, a mapping of the triple-frequency signatures to snowfall microphysical properties is possible for the first time. The triple-frequency observations reveal new opportunities to study snowfall microphysical processes and bear great potential to constrain our understanding of the scatter properties of frozen hydrometeors